Halogens refer to Group 17 in the periodic table consisting of the elements: fluorine (F), chlorine (Cl), Bromine (Br), Iodine (I) and Astatine (At). These elements are placed in this group because they have 7 valence electrons, a phenomenon which gives them a set of distinct properties.
Possessing 7 valence electrons would immediately make them very electronegative, as they would be more willing to accept an electron to complete their octet. For smaller halogens such as fluorine, since the electrons are closer to the protons in the nucleus, the pull/attraction towards a free electron is much stronger compared to a large halogen such as iodine.
Furthermore, since electron affinity is a great indicator for element reactivity, fluorine can be seen to be more reactive than chlorine, and chlorine more reactive than bromine. Due to this characteristically strong electron affinity, halogens have a proclivity to form acids when they are bonded to hydrogen. For example, HCl is a particularly strong acid, as chlorine’s affinity will remove the electron from the hydrogen and release a proton (H+). However, the main disadvantage of being reactive is that they are very toxic. This is why many halogens are used in disinfectants to kill any unwanted microorganisms
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